Two major conventional methods for the removal of organic contaminants from water are: (1) adsorption technologies employing granular activated carbon (GAC) or synthetic adsorbents, and (2) air stripping. A primary problem of both adsorption and air stripping processes is that they are nondestructive technologies, in that the organic contaminants are only transferred from one media to another, and are not destroyed to nontoxic matters. Therefore, the media into which the organic compounds are transferred, the GAC or synthetic adsorbent in adsorption processes and the air in air stripping processes, may require further treatment.
New regulations for air, water and solid residuals are stimulating researchers to search for new treatment technologies that either destroy or immobilize toxic organic compounds. Considerable research is being directed at effective and economical treatment strategies to meet the new standards. Advanced oxidation processes (AOPs) are alternative processes which destroy organic compounds and turn them into nontoxic forms, such as carbon dioxide (CO.sub.2) and water. AOPs involve the generation of highly reactive radicals, such as the hydroxyl radical (OH.), which are responsible for the destruction of the organic compounds. AOPs can be classified into two major groups: AOPs involving homogeneous reactions using hydrogen peroxide (H.sub.2 O.sub.2), ozone (O.sub.3), and ultraviolet (UV) light, alone or in any combination; and AOPs employing heterogeneous reactions using photoactive catalysts, such as semiconductors like titanium dioxide (TiO.sub.2). In the latter case--the photocatalytic oxidation processes--photoactive semiconductor catalysts are immersed in an oxygenated aqueous solution and illuminated with UV radiation, so that a redox environment is established which causes the oxidation of organic compounds.
The primary oxidant responsible for the photocatalytic oxidation of organic compounds in aqueous solutions is believed to be the highly reactive hydroxyl radical (OH.), although direct reactions of adsorbed organic compounds with surface species such as holes have also been reported Vo/ lz et al., 1981; Ceresa et al., Matthews, 1984; and Turchi and Ollis, 1990!. The proposed mechanisms and pathways for the formation of the hydroxyl radical have also been discussed in the literature. When a photoactive semiconductor is illuminated with photons of sufficient energy (band gap energy, or greater), a photon (h.nu.) excites an electron from the valance band, overcoming the energy band-gap to the conduction band, and leaves an electronic vacancy, a hole (h.sup.+), in the valance band. The band-gap energy is the minimum amount of energy required for exciting the electron. For example, anatase form TiO.sub.2 has a band-gap energy of about 3.2 eV, which is equivalent to the photon energy of UV light with a wavelength of 387 nm. Consequently, anatase form TiO.sub.2 can be activated by radiation with wavelengths less than 387 nm. The valance band hole and the conduction band electron may recombine either in the bulk of the semiconductor or at the external surface, producing heat or luminescence. Only the excited electrons and the resulting holes which survive the recombination may take part in the redox process with adsorbed species such as H.sub.2 O, OH.sup.-, organic compounds, and O.sub.2 at the water-solid interface. The holes may take part in the oxidation half reactions with adsorbed H.sub.2 O or OH.sup.- and form hydroxyl radicals. The electrons take part in the reduction half reactions with adsorbed O.sub.2 to produce the superoxide radical O..sub.2, which may also, in turn, produce H.sub.2 O.sub.2 and OH. Okamoto et al.,1985!.
More than 700 organic compounds have been identified in sources of drinking water in the United States Stachka and Pontius, 1984! and elsewhere. Many water utilities, companies and government agencies must also remove or destroy organic compounds from polluted groundwater supplies before those groundwater supplies can be used as drinking water. Additionally, many drinking water utilities are faced with the formation of disinfection by-products in finished water. Disinfection by-products are compounds formed in the water treatment process as a result of the disinfection step. In this step, a disinfectant such as chlorine is added to source water, where it reacts with a portion of the background organic matter (BOM) present in the source water to produce disinfection by-products. The reactive portions of the BOM are referred to as disinfection by-product precursors (DBP precursors).
U.S. Pat. No. 5,182,030 to Crittenden et al. discloses treating water by two separate steps using adsorbent materials impregnated with photocatalysts. In the first step, the photocatalyst-impregnated adsorbent materials are placed in a fixed bed to adsorb and remove organic compounds from contaminated water. In the second step, the impregnated adsorbent materials containing the adsorbed organic contaminants are regenerated by exposing them to UV radiation in a separate reactor.
U.S. Pat. No. 4,863,608 to Kawai et al. discloses a process step for preparing ultra pure water by purifying water including a small amount, particularly a trace amount on the order of several milligrams of carbon per liter (C/L) or less, of organic impurities included in total organic carbon (TOC). The process step is incorporated into known multi-step processes for the preparation of ultra pure water, the known processes comprising the steps of (a) one or more mechanical filtration steps followed by (b) a series of known purification steps essentially for the removal of residual solid particulates, such as reverse osmosis, adsorption on ion exchange resins, adsorption on activated carbon, ultrafiltration, UV sterilization, and microfiltration. The new step is incorporated into step (b) and comprises irradiating the water to be treated with light in the presence of a photocatalyst comprising an inorganic semiconductor selected from TiO.sub.2, SrTiO.sub.3 and CdS in fine particulate form and a noble metal and/or an oxide thereof selected from Pt, Pd, Ru, RuO.sub.2 and Rh deposited on the semiconductor particles for a period sufficient to oxidatively decompose the organic impurities. The new step results in a decrease in TOC content of the water to a level below the minimum detection level of TOC detectors, typically &lt;0.05 mg C/L or &lt;0.01 mg C/L depending upon the particular detector employed.
U.S. Pat. No. 4,861,484 to Lichtin et al. discloses a process for degrading organic compounds in a water-containing fluid. The fluid is combined with a peroxide and a solid catalyst comprising at least one transition element to form a reaction mixture, and photoenergy is added to the reaction mixture to yield environmentally compatible reaction products comprising at least carbon dioxide. The photoenergy is preferably in the visible or ultraviolet light ranges which are absorbable by the catalyst.